1. Field of the Invention
The present invention relates generally to image forming devices, and more particularly, to the sensing of toner density of deposited unfused toner in an image forming device.
2. Description of the Related Art
Electrostatically printed color images may be produced by depositing toners of various colors onto a recording media, such as a sheet of paper. Yellow, cyan, magenta and black toners in various proportions and combinations may be printed onto the recording media to produce a wide palette of printed colors. Each individual color of the producible palette may require a specific proportion and combination of toners. If the particular proportions of toner for a selected color cannot be repeatedly deposited on the printed media, then the printed color may not be consistent, and may vary in hue, chroma, and/or lightness from attempt to attempt of printing. The proportion of each toner color to be deposited may be based on the thickness of the toner layer of a given color. Therefore, controlling the printed colors, and ensuring reproducibility of the printed colors, may be achieved by controlling the toner layer thickness to ensure consistent color reproduction.
Toner patch sensors have therefore been used in printers and copiers to monitor the toner density of toner deposited onto a control surface in the printer, such as an intermediate transfer belt. Typically, such sensors may utilize a light source to illuminate a toner patch, and the reflectance of the incident light may be measured to indicate the thickness of the toner patch. The sensor may then provide a signal that the printer may use to adjust the toner density and control the print darkness. In color printers and copiers, toner patch sensors may be used to maintain the color balance and, in some cases, modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. The reflectivity of the toner patch may not be constant over time due to differences in the size of the toner particles, and the accumulation of toner resin and extraparticulate particles on the control surface. As a result, the reflectivity of the toner patch may not accurately indicate the amount of toner on the control surface.
Some printers, such as the LEXMARK C522, available from Lexmark International, Inc., may use algorithms that rely on the absolute voltage signal levels from the toner patch sensor to adjust the electrophotographic operating parameters in an attempt to control color density. Other printers, like the LEXMARK C750, also available from Lexmark International, Inc., may use algorithms that use only the ratio of the signal level of the test toner patch to the signal level of the bare belt. However, the reflectivity of the bare belt may not be constant over time due to the accumulation of toner resin, wax and extraparticulate particles on the belt, and, therefore, the reflectivity of the belt may not be accurately predicted. This problem may degrade the accuracy of the toner patch sensor in printers that use reflection ratios to monitor and adjust color print densities. The mechanical positioning and orientation of toner patch sensor components may also affect the magnitudes of the toner patch signal for both the bare belt and the test patches. Variations in these mechanical factors from one printer to the next often lead to degradation in the accuracy of the color control system for both the ratio method and the absolute voltage method of toner patch signal color control.
Known printers typically use a toner patch sensor that employs a single infrared wavelength for the measurements of toner reflectivity, typically around 880 to 940 nm. The use of a single wavelength and its associated hardware limits the accuracy of the toner patch sensor and the resulting calibration of the printer, as variations in the reflectivity of the control surface, the size of the toner particles, the accumulation of toner resin and extraparticulate particles all affect the accuracy of the calibration.